Polyolefin and other plastic molded articles are frequently used as members of automobile exterior panels, home appliances and the like. These molded articles are often pre-coated with a primer containing a chlorinated polyolefin and the like in order to improve adhesion between a top-coating and the molded article. Although toluene, xylene or other aromatic organic solvents have been used in this primer from the viewpoint of solubility of the chlorinated polyolefin, the use of aqueous primers has been promoted in recent years from the viewpoints of safety, health and protecting the environment.
Patent Document 1 and Patent Document 2 propose an example of an aqueous primer in the form of a composition comprising, at a specific ratio, a crosslinking agent and at least one type of aqueous resin selected from the group consisting of an aqueous polyolefin resin, an aqueous polyurethane resin and an aqueous acrylic resin for the purpose of forming a coating film that demonstrates superior adhesion with a polyolefin or other plastic molded article.
In addition, Patent Documents 3 and 4 propose an example of a 3-coat, 1-bake coating system, including top-coating, that consists of coating an aqueous primer followed by sequentially coating a colored base coating material and a clear coating material. In the case of using an aqueous primer in this type of wet-on-wet method, it is necessary to preliminarily dry (preheat) the aqueous primer after coating for, for example, several minutes prior to proceeding to the next coating step in order to ensure finish quality following top-coating.
However, actual coating lines are required to eliminate preheating steps and lower baking temperatures from the viewpoints of saving on space and energy, and as a result thereof, if the film thickness of the primer is reduced in an attempt to accelerate drying, a defective finished appearance may occur accompanying defective film formation depending on the environment in the coating booth (and particularly under low humidity conditions of a relative humidity of 65% or lower), and the additional problem may occur whereby it is not possible to secure electrical conductivity due to defective film formation. Moreover, in the case of multilayer coating films, it is difficult to eliminate the preheating step not only from the viewpoint of finish quality after top-coating, but also from the viewpoint of moisture resistance and other physical properties of the coating film.
Therefore, the present applicant proposed in Patent Document 5 that, by using an aqueous primer composition containing specific amounts of an aqueous dispersion of a modified polyolefin, an aqueous urethane resin and/or aqueous acrylic resin and a specific diester compound, a multilayer coating film can be formed that demonstrates superior finish quality and moisture resistance without preheating after coating and without exhibiting layer mixing even if top-coating is carried out in a subsequent step.